While in the past all piping was made of metal, usually metal, today plastic piping is in widespread use and has been found suitable for many applications. Plastic pipes are widely used for the protection of electric cables, sewage, drainage and, where the pipe diameter is small for water under pressure. Larger sizes of plastic piping are suitable for conditions where there is little or no internal pressure, and in applications not exposed to solar radiation. The present invention relates particularly to intermediate size piping, typically the 50 to 200 mm range. Many prior art designs are suitable for pipes under about 5o mm, while the over 200 range is served well by band clamps. The present invention refers to a gripper foot where the same clamp can be usefully applied to fittings carrying metal or plastic pipes.
As far as the present invention is concerned it should be noted that not only are metals harder than plastics, but also metals have a Young's Modulus about 10 times higher than plastics. Thus if a pointed tool is pressed onto the outside of a plastic pipe the tool will penetrate deeper and distort the pipe to a much greater degree than if the same tool and force was applied to a steel pipe having the same dimensions. Thus a prior art pipe coupling for example suitable for a 3″ steel pipe will be unsuitable for a 3″ pipe of the same wall thickness when made of a plastic.
These considerations are extended also to applications such as material feeding as part of a manufacturing process. The grip unit is useful for feeding sheet material to a press which is used sometimes for manufacturing metal articles and at other times for manufacturing articles made from materials other than metals.
The following brief review of recent U.S. patents provides an assessment of the state of the art.
In U.S. Pat. No. 6,019,396 Saito et al. disclose a pipe coupling having a retainer member which is axially compressible to resist an axial force separating the pipes. Due to a complex outer shape two different degrees of compression are available, in order to handle both plastic and steel pipes. However there is some doubt regarding retention of steel pipes as the retainer member does not penetrate the outer skin of the pipe.
DeMore et al. claim in U.S. Pat. No. 6,106,029 that their design of pipe coupling has improved pull-out resistance for plastic and for steel pipes. The lock ring used requires several arcuately-spaced slots for slideably supporting individual gripper segments, which are claimed to effect a penetrating restraining grip against the pipe end. As said gripper makes line contact with a steel pipe very high pressure would be required to penetrate the pipe surface.
A similar disadvantage is present in the coupling disclosed by Hulsechos et al. in U.S. Pat. No. 6,481,755. Some increase in holding power is provided by dents pressed into the grip ring, which is a complex and costly structure.
The complex pipe coupling disclosed by Robinson, Jr. in U.S. Pat. No. 6,565,125 features the well-known axial-movement conical lock. The design includes a bolt having an axis tangential to the pipes being held in the coupling intended to limit the force applied by the clamp collar. The bolt thus requires adjustment when set to a hard material while a soft material is being gripped and also when set to a soft material pipe while a hard material is being gripped. The risk is high that during installation the bolt will be wrongly adjusted.
Isenam et al. propose a high grip coupling in US Patent Application No. 2002/0101080. This design requires machining and swaging one of the pipe ends, which is impractical in the field and even in the factory is difficult for long pipes.